Smartjack for fault-testing network segments on Ethernet and other internet protocol network architectures

ABSTRACT

A fault-testing node for a connectionless data link has at least two opposing communication ports; a soft switch for controlling port-to-port data flow through the device; and an instance of software for modifying packet header information. The node enables loop-back testing by one or more port-to-port data flow paths being switched through activation of the soft switch to loop incoming packets back to the sender of the packets through the device and wherein the instance of software reverses the order of source and destination addresses of data units to insure acceptance of looped data units at the sender station.

FIELD OF THE INVENTION

The present in invention is in the field of data packet communicationover a data packet network and pertains more particularly to acomputerized device functioning as a controlled demarcation point fordata packets in a data packet network used for fault testing and othernetwork testing.

BACKGROUND OF THE INVENTION

A conventional Digital Subscriber Line (DSL) leased from atelecommunications company uses a Time Division Multiplex (CDM) carrieror circuit, over which connection-oriented digital communication ispracticed The span of cable or line extending between thetelecommunications provider and the Customer Premise Equipment (CPE) issometimes referred to in the art as a local loop. The first precursor toDSL technology was Integrated Services Digital Network (ISDN), whichunlike pure DSL, transmits data as analog data over a telephone line butuses analog-to-digital (A/D) conversion for receive at the user end anddigital-to-analog (D/A) conversion for send at the user end with an ISDNmodem. Pure DSL does not require any data transformation between analogand digital. All transmissions over the phone line are digital and DSLcable modems are used.

Multiple DSL users are typically connected to a Digital Subscriber LineAccess Multiplexer (DSLAM), which is in turn connected to a high-speednetwork backbone, typically an Asynchronous Transfer Mode (ATM)backbone. A DSLAM also de-multiplexes data traveling from the backboneto other geographically disparate destination loops or circuits. Othertypical and emerging DSL technologies include Asymmetric DigitalSubscriber Line (ADSL), Symmetric Digital Subscriber Line (SDSL), Veryhigh-speed Digital Subscriber Line (VDSL), and High-bit rate DigitalSubscriber Line (HDSL). Most typical home users are familiar with ADSL.

In a TDM connection-oriented network, fault testing a DSL segment orline is typically enabled by a mechanism known in the art as asmartjack. A smartjack is typically implemented in known art, forexample, over an HDSL line using T1/DS1 protocols. T1/DS1 is a digitalsubscriber (DS) level and data framing specification for synchronousdigital streams at a T1 transmission rate of 1,544,000 bits per second.

A smartjack is an input/output (I/O) device running logic, such assoftware, that is installed between the telecommunications switch(Telco) and a CPE switch or router in a local loop on the customer end.A typical smartjack is a box that has a Telco-side RX/TX port and acustomer or CPE-side RX/TX port.

A smartjack is activated remotely by an administrator when link testingis to be performed. When in test mode the smartjack is caused tophysically loop every bit received on the RX link of a port to thecorresponding TX link of the same port. The data frames sent to the boxare received back over the same line without any modifying of thestructure. The loop-back testing technique is a standard and is wellknown in the art as a method of determining whether a detected networkfault is somewhere in the transmission network or on the customer siteof a connection-oriented link When the device is not in test mode, itremains in a transparent pass through mode where every bit received (RX)on one port is sent on through to the transmit line (TX) on the oppositeport the box transmitting data as such bi-directionally.

While a smartjack of prior-art enables fault testing and isolation on aTDM connection-oriented data link, it cannot be used with newerpacket-switched or connectionless network lines, for example those usingTransfer Control Protocol/Internet Protocol (TCP/IP) including Ethernetnetwork technologies. For example, if a data packet were transmitted toa prior-art smartjack installed in a connectionless link, and it were tobe returned to the source station unchanged from the same port it wouldbe rejected by the sending station as not being addressed to the sendingstation. This is because data packet headers (IP or Ethernet) must beaddressed for source (sender) and destination (receiver) according torequest/response protocols.

If the destination address header field of a data packet for send is setto the address of the sending station while at the sending station thenthe packet could not be sent. In other words, data loops are notpermissible in a connectionless architecture. Attempting a softwareworkaround to this problem is also not productive, because if the packetcould be transmitted from the sending station having the destinationaddress of the sending station then the software at the CPE switch orrouter could not properly forward the packet.

Therefore, what is clearly needed is a fault-testing device somewhatsimilar to a smartjack that can provide the same fault testing servicesusing a point-to-point tunneling protocol for a packet-switched networkline.

SUMMARY OF THE INVENTION

In an embodiment of the present invention a fault-testing node for aconnectionless data link is provided, comprising at least two opposingcommunication ports, a soft switch for controlling port-to-port dataflow through the device, and an instance of software for modifyingheader information associated with data units, wherein one or moreport-to-port data flow paths are switched by activating the soft switchto loop incoming data units back to the sender of the data units throughthe device, and wherein the instance of software reverses the order ofsource and destination addresses of data units to insure acceptance oflooped data units at the sender station.

In one embodiment the connectionless data link is an Ethernet data link,the data units are Ethernet frames, and the source and destinationaddresses are MAC addresses. In another embodiment connectionless datalink is an IP data link, the data units are IP data packets, and thesource and destination addresses are IP addresses. In some embodimentsthe instance of software modifies the data unit header fields by copyingthe data from the first field to memory, copying the data from thesecond field and pasting the data into the first field, and then pastingthe data from the memory into the second field.

In some embodiments there are two operating modes, a loop-back mode anda pass-through mode. There may further be an array of resident faulttesting applications and a logging component to create activity anderror logs during testing and during normal operation.

Also in an embodiment loop-back tests performed include one or acombination of tests returning data for number of bytes sent or receivedover the link for a specified period, number of packets sent or receivedover the link for a specified period; number of CRC errors occurringover a specified period, average packet length of packets sent orreceived over the link during a specified period, average transmissionrate over the link address identification of the link and protocol typesin operation over the link.

In some embodiments intrusive tests are performed includingBit-Error-Rate-Testing, testing for throughput between the node andanother on-line device, testing for packet delay between any two pointson the link, testing for jitter between any two point on the link, andtesting for packet loss between any two points on the link.

In another aspect of the invention, in a fault-testing node for aconnectionless data link, the node having at least two opposingcommunication ports and an instance of software for modifying data unitheader information, a method is provided for switching data setsresident in address fields of a data unit, enabling loop-back of dataunits received at the node to the sender of the data units. The methodcomprises steps of (a) copying the data set from a first field to amemory; (b) copying the data set from a second field; (c) pasting thedata set from the second field into the first field; and (d) pasting thedata set copied to memory in step (a) into the second field.

In some embodiments of the method in step (a) the first field is asource address field the data set a source IP address of an IP datapacket. Also in some embodiments in steps (b) and (c) the second fieldis a destination address field the data set a destination IP address,which becomes a source IP address when pasted into the source field ofthe IP data packet.

In some embodiments in step (d) the data set is a source address, whichbecomes a destination IP address when pasted into the destination fieldof the IP data packet. Also in some embodiments in step (a) the firstfield is a destination address field the data set a destination IPaddress of an IP data packet. In some other embodiments in steps (b) and(c) the second field is a source address field the data set a source IPaddress, which becomes a destination IP address when pasted into thedestination field of the IP data packet.

In still other embodiments of the method in step (d) the data set is adestination IP address, which becomes a source IP address when pastedinto the source field of the IP data packet. Also in other embodimentsin step (a) the first field is a source address field the data set asource machine access code address of an Ethernet data frame. In stillother embodiments in steps (b) and (c) the second field is a destinationaddress field the data set a destination machine access code address,which becomes a source machine access code address when pasted into thesource field of the Ethernet data frame.

In yet other embodiments of the invention in step (d) the data set is asource address, which becomes a destination machine access code addresswhen pasted into the destination field of the Ethernet data frame. Alsoin other embodiments in step (a) the first field may be a destinationaddress field the data set a destination machine access code address ofan Ethernet data frame. Also in embodiments in steps (b) and (c) thesecond field may be a source address field the data set a source machineaccess code address, which becomes a destination machine access codeaddress when pasted into the destination field of the Ethernet dataframe. In still other embodiments in step (d) the data set is adestination machine access code address, which becomes a source machineaccess code address when pasted into the source field of the Ethernetdata frame.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 is an overview of a telecommunications network using anIP/Ethernet smartjack according to an embodiment of the presentinvention.

FIG. 2A is a block diagram illustrating components of the smartjack ofFIG. 1 operating in a pass-through mode.

FIG. 2B is a block diagram illustrating components of the smartjack ofFIG. 1 operating in a loopback mode.

FIG. 3 is a process flow diagram illustrating basic steps for data flowthrough the smartjack of FIG. 1 during a loop-back mode according to anembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The inventor provides a packet demarcation device (an enhancedsmartjack) that is supported in a connectionless TCP/IP networkincluding an Ethernet network, and may also be useful in otherconnectionless networks. The methods and apparatus of the invention aredescribed in enabling detail below with reference to embodiments of theinvention.

FIG. 1 is an overview of a telecommunications network 101 hosting anIP/Ethernet smartjack 113 according to an embodiment of the presentinvention, and a customer premise 102. CPE site 102 can be an enterprisehosting an Ethernet Local Area Network (LAN) or Wide-Area-Network (WAN),or a network adapted according to standard Transfer ControlProtocol/Internet Protocol (TCP/IP) and other Internet protocols.

CPE 102 in this example has an Ethernet or IP-enabled LAN 110 that isused to interconnect equipment responsible for company networkcommunication. LAN 110 in this example has a plurality of computerworkstations 108 a–n connected thereto for communication and resourcesharing. Workstations 108 a–n represent typical agent workstations of anenterprise. Telephones 109 a–n are illustrated within CPE 102 and arelogically illustrated as connected to LAN 110. It may be assumed thatthere exists one telephone 109 a–n for each workstation 108 a–n). Theinventor illustrates only two such telephones and deems the illustrationsufficient for the purpose of explaining the present invention. Of theexemplary equipment types illustrated in this example as connected toLAN 110 are a shared printer 111, and a shared facsimile machine 112.

Network 110 is in one example an Ethernet network, but also may be an IPprotocol network, although there are some overlapping features. A CPEswitch/router (SW/RTR) 107 is provided in this example within CPE 102and is adapted to perform bi-directional routing of interactionsincoming to and outgoing from network 101. For example, all dataincoming to RTR 107 is forwarded along over network 110 to appropriatestations 108 a–n. RTR 107 represents the last hop in the network beforeLAN 110. Similarly it is the first hop for outgoing messages sourcedfrom any machine connected to LAN 110.

IF LAN 110 is an Ethernet network then the Ethernet protocols (FaringTechnology), Machine Access Codes (MAC addresses), an applicableEthernet hub, and Ethernet network cards with an Ethernet driver aretypically used according to standard Ethernet network set-up andprotocols. If LAN 110 is not Ethernet, but an IP-enabled LAN (PacketBased), then all of the normal and standard IP conventions are observed.One difference in function between Ethernet and non-Ethernet packetswitched networks relates to the present invention in how data ispackaged. Ethernet uses a packet framing technology (packet frames) thatemploys machine code addresses (MAC) in a request/response format, forexample, a source MAC address and a destination MAC address. IP uses adata packet format utilizing a source IP address and a destination IPaddress for request/response communication.

Telecommunications network 101 encompasses in this example atelecommunications host or carrier (Telco) switch 103 connected to anetwork backbone 105. Telco 103 represents a local switch site facility,which may include other standard equipment (not shown). Backbone 105 canbe an ATM backbone without departing from the spirit and scope of thepresent invention. Likewise, other equipment types and carrier lines maybe assumed present in network domain 101.

A service provider that provides telecommunications services carrieddigitally over connection-oriented serves as a host in this example forTelco switch or facility 103 and, in this case as well, connectionlessleased lines or trunks to end users or subscribers. An edge router 104is illustrated within the domain of network 101 and marks the edge ofthe network where data departing enters a disparate network or loop anddata arrives from a disparate network or loop. Router 104 is typicallythe last routing point for data destined for CPE 102.

IP/Ethernet Smartjack 113 in this example is a packet demarcation pointbetween Telco equipment and lines, and CPE equipment and lines.Smartjack 113 in this example is owned and provided by Telco 103 as alocal service provider. Smartjack 113 provides a network fault-testingfacility that is capable of loop-back packet transmission utilizingeither the network or customer-side port. Smartjack 113 in oneembodiment uses a novel packet loop-back technique enabled by logic,typically software, (not illustrated) running on the device. Anadministrator on the Telco side of smartjack 113 can carry out a seriesof automated and intrusive tests by calling smartjack 113 andinstructing it to initiate and perform testing by sending instructiondata to it in a data packet or frame (Ethernet). The administrator cancheck the line between backbone 105 and jack 113. Likewise anadministrator on the side of CPE 102 operating from LAN 110, forexample, can test network function and performance on the customer-side.

As was described above in the background section, newer packet-basednetworks including Ethernet and IP networks, package data in datapackets (IP) or Ethernet frames (Ethernet) wherein source anddestination addresses are used in a sequence-based request/responseformat. Therefore, the smartjack of prior-art will not work in thisembodiment because it must physically loop (hard switch) all datareceived at a port on a receive wire back out the same port on acorresponding transmit wire with the data absolutely unchanged.

Smartjack 113, unlike devices of prior ark is enhanced with logic,typically software, but may be firmware, hardware, or a combination,that is adapted to swap source and destination addresses in packetheader address fields enabling a received packet to arrive at a port anddepart out of the same port back to the sender whether the sender is onthe Telco side or the customer side. The ability to perform IP andEthernet loop-back testing is described in more detail below in thisspecification.

FIG. 2A is a block diagram illustrating an embodiment with components ofsmartjack 113 of FIG. 1 operating in a transparent pass-through mode.Smartjack 113 in this example has at least two communication portslabeled herein as a Telco port 200, and a Customer port 201. Each port200 and 201 is enabled to RX (receive) packets and to TX (transmit)packets.

Smartjack 113 in this example has a processor 202 adapted with logic forperforming basic functions including memory caching and data unitprocessing. Processor 202 in this example has an array of fault testinglogic applications 203 that can be remotely initiated and programmed toperform certain tests. Processor 202 has an archiving or loggingsoftware 207 associated with applications 203 adapted to record all dataactivity and results during testing and normal runtime includingcompiling and maintaining data history, statistics, and error logs asmight be related to any one or a combination of test procedures ornormal data flow-through.

A communication stack 204 is provided in this embodiment on processor202 and adapted for data unit generation and send for any unitsoriginating from smartjack 113 containing test results, history data,error logs, and so on. If smartjack 113 is adapted for TCP/IP, then thedata units are data packets. If the smartjack is adapted for Ethernetthen the data units are Ethernet data frames. Smartjack 113 in thisembodiment is a communication node and is capable of alerting remotenodes of status, and is also capable of requesting communication withremote nodes using TCP/IP or Ethernet protocols. If smartjack 113 wereadapted as an Ethernet smartjack then it would have a network card (notillustrated), a MAC address burned thereon and be powered by theEthernet network driver.

At the heart of smartjack 113 in this embodiment is a soft switch, whichcan be remotely commanded and controls port-to-port packet flow throughthe device. When smartjack 113 is not in loop-back mode in thisembodiment, it is in a pass-through mode transparent to the customer. Inpass-through mode for example, all data units received at Telco port 200that are destined for a CPE-based entity (referring to FIG. 1, LAN 110,machines 108 a–n) may be forwarded to TX on customer port 201representing normal data flow in one direction (solid directionalarrows). Likewise, all data units received at customer port 201 destinedfor entities outside of CPE domain 102 may be forwarded to TX on Telcoport 200 (dotted directional arrows). In this mode jack 113 iscompletely transparent.

In pass-through mode all data is copied to processing unit 202 foranalysis. Analysis of all data moving through smartjack 113 providesrequired information about data unit activity, error states, and otherstatistics about network performance. Smartjack 113 in this embodimenthas a logic instance 205 labeled herein Loop-Back Software. Logic 205provides a capability for swapping source and destination addresses fordata units that are to be looped back to a sender. More detail about howdata loop-back is accomplished is provided below.

FIG. 2B is a block diagram illustrating components of smartjack 113 ofFIG. 1 operating in a loop-back mode according to an embodiment of thepresent invention. All of the elements described in the example of FIG.2A above are present also in this example. Therefore, the elementscommon to both examples retain the same element numbers as in FIG. 1 andare not reintroduced.

In this example smartjack 113 is in loop-back mode. Loop-back mode is amode that may be configured on the fly or on a schedule for the purposeof looping back all data received by smartjack 113 back to the senderout of the same port for testing purposes. A network administrator orother authorized agent operating a network-connected node may enableloop-back mode by sending a data unit (packet or frame) containing theinstruction set for configuring smartjack 113 to begin looping backdata. For example, a network administrator or some authorized agent onthe Telco side of smartjack 113 might initiate a data unit that containsinstructions for initiating loop-back mode. After receiving the dataunit, soft switch 206 is automatically configured to begin looping backdata.

After smartjack 113 is configured for loop back mode, all data receivedat Telco port 200 (RX) will be retransmitted back to the sender throughTX on the same port. Likewise all data received (RX) at customer port201 will be retransmitted back to the sender through TX of port 201. Inthis regard, both sides of the line, the network side and the customerside can be fault tested sequentially or simultaneously.

It is noted herein that all data is copied and analyzed by processor 202at the same time as fault testing so activity logging is notinterrupted. Loop-back logic 205 functions in this embodiment to workwith the header address fields of each received data unit duringloop-back mode so that the source and destination addresses are reversed(swapped) such that the sender address becomes the destination addressfor transmission out of the same port. Normal header address fields inthis example are placed at fixed offsets from the start of the data unitwhether the unit is a data packet (IP) or a data frame (Ethernet).Software 205 copies the first address into temporary memory (notillustrated) provided on processor 202. The second address is thencopied and pasted over the first address. The address in temporarymemory is then copied over the second address. The data unit must berecalculated for Cyclic Redundancy using a Cyclic Redundancy Check (CRC)scheme, which may vary from scheme to scheme according to prevailingprotocol.

A significant advantage of smartjack 113 in the loop-back mode is thatfaults can be quickly detected and isolated as either existing at somepoint on the Telco side of the connection or on some point on the CPEside of the connection. It is noted herein that most fault testing overa connectionless network line using smartjack 113 is performed in layer2 of the OSI model using a tunneling protocol. If smartjack 113 isplaced on an Ethernet network then source and destination MAC addressesare swapped by software 205. In an IP network connection, IP source anddestination addresses are swapped. The typical communication stack 204does not have to be modified in order to practice the present invention.Logic 205 sits on top of communication stack 204.

Loop-back testing can return information about the link being tested,such as the number of bytes sent or received on the line over aspecified testing period; the number of data units sent or received overthe line; the number of CRC errors occurring over a specified period;average length of data units sent or received; average transmission rateof the line; address information seen on the link; current protocoltypes being utilized over the link; and for Ethernet, the number of runtdata frames occurring over an Ethernet link Runt data frames areillegally small frames usually caused by frame collision on ahalf-duplex Ethernet link.

It is also noted herein that smartjack 113 in certain embodiments can beconfigured on the fly to initiate and perform a number of differentintrusive tests at varying protocol levels. For example, smanijack 113may be configured to run a Bit-Error-Rate-Test (BERT) usingPsuedo-Random-Bit-Sequence (PRBS) patterns or any user-defined patterns.A BERT test is typically used to test the ratio of erroneous bitsreceived on the link for example to the number of received bits total.In addition, throughput between smartjack 113 and any other on-linedevice may be determined as well as delay, jitter, and packet/frame lossbetween any two points on the network. It is also noted herein that theabove-described tests may be ordered and performed according to variouscombinations and sequences including multiple instantiations of the sametest.

FIG. 3 is a process flow diagram illustrating basic steps for data flowthrough the smartjack of FIG. 1 during a loop-back mode according to anembodiment of the present invention. Assuming that smartjack 113 isconfigured and ready for loop-back testing, at step 301 a data unit,which may be an IP packet or Ethernet frame, is received at either theTelco, or customer side port depending on the location of the testadministrator. At step 302 a, the loop-back software copies the firstaddress found in the unit header to temporary memory. Simultaneously, atstep 302 b data of the received data unit is copied for the purpose ofprocessing and analyzing.

At step 303 the loop-back software copies the second found address fromthe unit header and pastes it into the first address field at theappropriate offset position. At step 304 the loop-back software pastesthe first address into the second address field. Now the data unit isaddress-ready for retransmission out of the same port it was received.

At step 305, the processing component attaches any appropriate data tothe data unit payload such as return of test results, time stamps, andso on. Because the data unit was altered, at step 306 a CRC is performedon the unit. At step 307 the data unit is transmitted out of the sameTelco or customer port that it was received on with the destinationaddress being that of the original sender.

It will be apparent to one with skill in the art that the methods andapparatus of the present invention can be applied to Ethernet links orIP links over a WAN or LAN architecture without departing from thespirit and scope of the present invention. Using the address-swappingtechnique enables fault identification and geographic isolation on bothsides of a leased line, for example on the Telco side and on thecustomer side.

The methods and apparatus of the invention should be afforded thebroadest possible scope under examination. The methods and apparatus ofthe present invention are limited only by the claims that follow.

1. A fault-testing node for a connectionless data link comprising: atleast two opposing communication ports; a soft switch for controllingport-to-port data flow through the device; and an instance of softwaremodifying header information associated with data units by copying thedata from the first field to memory, copying the data from the secondfield and pasting the data into the first field, and then pasting thedata from the memory into the second field; wherein one or moreport-to-port data flow paths are switched by activating the soft switchto loop incoming data units back to the sender of the data units throughthe device, and wherein the instance of software reverses the order ofsource and destination addresses of data units to insure acceptance oflooped data units at the sender station.
 2. The node of claim 1 whereinthe connectionless data link is an Ethernet data link, the data unitsare Ethernet frames, and the source and destination addresses are MACaddresses.
 3. The node of claim 1 wherein the connectionless data linkis an IP data link, the data units are IP data packets, and the sourceand destination addresses are IP addresses.
 4. The node of claim 1having two operating modes, a loop-back mode and a pass-through mode. 5.The node of claim 1 further comprising an array of resident faulttesting applications and a logging component to create activity anderror logs during testing and during normal operation.
 6. The node ofclaim 1 wherein loop-back tests performed include one or a combinationof tests returning data for number of bytes sent or received over thelink for a specified period, number of packets sent or received over thelink for a specified period; number of CRC errors occurring over aspecified period, average packet length of packets sent or received overthe link during a specified period, average transmission rate over thelink, address identification of the link, and protocol types inoperation over the link.
 7. The node of claim 1 wherein intrusive testsare performed including Bit-Error-Rate-Testing, testing for throughputbetween the node and another on-line device, testing for packet delaybetween any two points on the link, testing for jitter between any twopoint on the link, and testing for packet loss between any two points onthe link.
 8. In a fault-testing node for a connectionless data link, thenode having at least two opposing communication ports and an instance ofsoftware for modifying data unit header information, a method forswitching data sets resident in address fields of a data unit enablingloop-back of data units received at the node to the sender of the dataunits comprising steps of: (a) copying the data set from a first fieldto a memory; (b) copying the data set from a second field; (c) pastingthe data set from the second field into the first field; and (d) pastingthe data set copied to memory in step (a) into the second field.
 9. Themethod of claim 8 wherein in step (a) the first field is a sourceaddress field the data set a source IP address of an IP data packet. 10.The method of claim 9 wherein in steps (b) and (c) the second field is adestination address field the data set a destination IP address, whichbecomes a source IP address when pasted into the source field of the IPdata packet.
 11. The method of claim 10 wherein in step (d) the data setis a source address, which becomes a destination IP address when pastedinto the destination field of the IP data packet.
 12. The method ofclaim 8 wherein in step (a) the first field is a destination addressfield the data set a destination IP address of an IP data packet. 13.The method of claim 12 wherein in steps (b) and (c) the second field isa source address field the data set a source IP address, which becomes adestination IP address when pasted into the destination field of the IPdata packet.
 14. The method of claim 13 wherein in step (d) the data setis a destination IP address, which becomes a source IP address whenpasted into the source field of the IP data packet.
 15. The method ofclaim 8 wherein in step (a) the first field is a source address fieldthe data set a source machine access code address of an Ethernet dataframe.
 16. The method of claim 15 wherein in steps (b) and (c) thesecond field is a destination address field the data set a destinationmachine access code address, which becomes a source machine access codeaddress when pasted into the source field of the Ethernet data frame.17. The method of claim 16 wherein in step (d) the data set is a sourceaddress, which becomes a destination machine access code address whenpasted into the destination field of the Ethernet data frame.
 18. Themethod of claim 8 wherein in step (a) the first field is a destinationaddress field the data set a destination machine access code address ofan Ethernet data frame.
 19. The method of claim 18 wherein in steps (b)and (c) the second field is a source address field the data set a sourcemachine access code address, which becomes a destination machine accesscode address when pasted into the destination field of the Ethernet dataframe.
 20. The method of claim 19 wherein in step (d) the data set is adestination machine access code address, which becomes a source machineaccess code address when pasted into the source field of the Ethernetdata frame.